I believe in interviews it was said that if vaughn took on secret service he would have to start straight away and lets be honest vaughn wanted to do a james bond style film so much that he brought some of that to Xmen first class so chances are thats why given the chance he jumped on to secret service, but he is still a producer on DOFP and F4

vaughn may have taken a few liberties here and there but he did a great job with FC, he brought a fresh approach to the film while staying pretty faithful to singers vision, alot more closer then Origins did, even replicating the first scene from X-men step by step